Good adhesion properties, particularly to chalky substrates, are important to coatings such as paints and architectural coatings. One conventional method for improving the chalk adhesion properties of a coating composition is to add an alkyd resin to the conventional coating composition. However, alkyd resins produce premature yellowing, rendering the coatings undesirable in certain circumstances.
Another method for improving the chalk adhesion properties is to synthesize a polymer blend in a sequential polymerization. Examples of acrylic-based polymer blends made by sequential polymerization processes can be found in U.S. Pat. Nos. 5,990,228 and 6,710,112 B1.
The '228 patent discloses aqueous coating compositions containing at least two polymer components which supposedly provide adhesion and improved durability, as measured in dried coatings made from the aqueous compositions by improved gloss retention or dirt pickup resistance. Specifically, in Comparative Example 1 of the '228 patent, this patent discloses the use of a pre-polymerized latex core of 60 nm particulate poly(butyl acrylate-co-methyl methacrylate-co-methacrylic acid), upon which was further polymerized two layers of poly(butyl acrylate-co-methyl methacrylate-co-acrylic acid) of the same composition. The at least two polymer components are synthesized as core-shell polymers by a sequential polymerization process.
The '112 patent discloses aqueous polymer dispersions having two polymer phases that have different glass transition temperatures but that are formed by a sequential polymerization process. In addition, the molecular weights of the two polymer phases are different, due to the addition at some point during the sequential polymerization of a chain transfer reagent.
Other publications disclose alternatives to alkyd resin modification in coating compositions.
For example, U.S. Patent Application Publication No. 2004/0161542 A1 and U.S. Pat. No. 6,630,533 both disclose compositions containing at least one fatty acid ester. In the '542 publication, the fatty acid ester is unsaturated. In the '533 patent, the fatty acid ester comprises a C12 to C40 alkyl (meth)acrylate.
U.S. Pat. No. 5,376,704 discloses aqueous coating compositions containing a neutralized half-ester product of an acrylic polymer containing at least two reactive anhydride groups that is crosslinked with an epoxy crosslinker. The molecular weights of both these components are less than 100,000 Daltons.
U.S. Patent Application Publication No. 2004/0010091 A1 discloses two component coating compositions that cure under ambient conditions. The coating composition of the '091 publication contains crosslinkable and crosslinking components, with both components having molecular weights under 100,000 Daltons.
U.S. Patent Application Publication No. 2004/0010071 A1 discloses an aqueous polymer blend composition containing soft and hard polymer particles. The hard polymer particles have a glass transition temperature greater than 25° C., while the soft polymer particles have a glass transition temperature less than or equal to 25° C. In addition, the hard polymer particles constitute from 2-30 wt % of the composition, while the soft polymer particles constitute from 70-98 wt % of the composition.
U.S. Patent Application Publication No. 2005/0009954 A1 discloses aqueous polymer compositions containing hard polymers and soft polymers, with at least the hard polymers include phosphorus-containing and/or polyacid-containing pendant groups. The hard polymers have a glass transition temperature of at least 20° C., while the soft polymers have a glass transition temperature of from −20° C. to 5° C. In addition, the hard polymers constitute from 2-40 wt % of the composition, while the soft polymer particles constitute from 60-98 wt % of the composition.
There remains a need for other varied alternatives to alkyd resins for improving adhesion.